A couple of possibilities.. If you have primer residue in the pocket it can cushion the hammer fall enough to keep you from having proper ignition.

I had a series of failures, and tracked it down to a tray of primers that spent the winter in my unheated damp shop.

Over time I have had ignition problems, one was traceable back to gun oil in the firing pin channel of a 1911 Colt, the oil migrated down onto the primer, the case was one that had been reloaded several times and the primer pocket was a bit loose.

I have had condensate in cases that caused problems, usually from using uncleaned scrounged brass.

Another culprit is case tumbling media, that can lodge in the flash hole.

I suspect it was powder contamination in one case due to neglect to properly clean lube from the base of a bullet.
I pan lube and therefore need to wipe the bases of most of the bullets I load. I would guess that I got in a hurry and left a chunk of lube on the base that fell into the powder. That sounds pretty iffy but is the best guess that I have right now. Primer pocket residue is a real possibility with a Dillon 550.
At any rate I've abandon the .38 cases and went back to my original load of 14 grains 2400 in a .357 case.
It's been a long winter and I've played with many variations to kill the boredom.

John, I won't abandon that 6 grain load for practice though my S&W tends to lead and Unique seems to be much hotter temperature wise than 2400. Mr. Keith tried to explain that to me once and I wasn't sure but he was proven right again. I will lead more with any Unique load than a load with 2400.

I do not recall when I last saw a can of 2400 at my local dealer, I bought a couple of cans of 4227, which I think for your application might be a better choice than the 2400.

I have shot a lot of 170 gr. swc bullets over 14.5 gr. of 4227, it is one of those powders that needs to be loaded right up near maximum for good uniform burning.

The above load with the 358429 bullet is a compressed charge, it very accurate. With anything other than maximum loads I have always had unburned powder with 2400
and the above 4227 load has about the same velocity as the upper end 2400 load with a bit lower pressure.

John, 2400 is pretty common here. I like it for the heavier loads and have quite a bit of HP38 that was all I could find during the shortage so I'm trying to get a nice lite practice load with it.
Snowed a bit today with a high of 25. Hard to get much shooting done.

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